[llvm] [BOLT] Add 'constant island' check in scanExternalRefs to prevent a crash when the function is disassembled fail or skipped (PR #165577)

[Maksim Panchenko via llvm-commits]

https://github.com/maksfb approved this pull request.

Looks good - thanks! I would refactor code to eliminate code duplication by introducing something like `BC.handleCodeRef()`, but it's not a hard requirement and I can do it once you commit the change.

Please shorten the title before the commit.

https://github.com/llvm/llvm-project/pull/165577